An electron source is a device or component capable of generating electron beam currents, often called electron gun, cathode, emitter, etc. Electron sources are widely used in displays, X-ray sources, microwave tubes, etc. An X-ray source is a device that generates X-ray. The core part of the X-ray source is an X-ray tube. The X-ray source comprises an electron source, an anode and a vacuum seal housing, and usually further comprising a power supply, a control system and auxiliary components, such as a cooling, a shield and so on. The X-ray source is widely used in industrial nondestructive testing, security check, medical diagnosis and treatment, etc.
Traditionally, an X-ray source adopts a direct cooling tungsten filament as the cathode. During operation, the filament through which an electric current flows is heated to an operating temperature of about 2000K and then generates an electron beam current through thermal emission. The electron beam current is accelerated by an electric field at hundreds of thousands of voltage between the anode and the cathode toward the anode, strikes a target and then generates an X-ray.
Field emission can be caused by a plurality of materials, such as metal needle, carbon nano-tube, etc., to emit electrons at room temperature and generate electron beam currents. After the development of nanotechnology, especially carbon nano-material, field emission electron sources with nano-materials grow quickly.
An X-ray source requires its electron source to have a large emission current, usually larger than 1 mA. For example, in existing medical CTs, oil-cooled X-ray sources with rotating targets can emit an electric current of up to 1300 mA. As disclosed in Patent Reference 1, in an existing X-ray device which adopts a field emission electron source with nano-material as cathode, in order to obtain a large emission electric current, a cathode emission surface with a macro size is formed from nano-material, and a mesh grid is arranged above and in parallel with the emission surface to control the field emission. In such structure, due to machining accuracy, deformation of the mesh and installation accuracy, there is a large distance between the mesh grid and the cathode surface, thus the grid needs a very high voltage, normally larger than 1000V, to control the field emission.
Usually, electron emission units using the field emission principle have the substantially same structure, for example, as shown in parts (A), (B) and (C) of FIG. 3. Part (A) of FIG. 3 shows the technical solution disclosed in Patent Reference 2, wherein a nano-material 31 is adhered to a structure 13 of a substrate 10. Part (B) of FIG. 3 shows the technical solution disclosed in Patent Reference 3, wherein a nano-material 20 is directly formed on flat surfaces of substrates 12 and 14. Part (C) of FIG. 3 shows the technical solution disclosed in Patent Reference 4, wherein an electron source for an X-ray source device comprises a nano-material surface 330 with a micro size (millimeters to centimeters), and its grid is a mesh grid with a micro size, and the grid surface is parallel to the nano-material surface.    Patent Reference 1: CN102870189B;    Patent Reference 2: U.S. Pat. No. 5,773,921;    Patent Reference 3: U.S. Pat. No. 5,973,444; and    Patent Reference 4: CN100459019.